ACR MLG FAILED TO PRESSURIZE.

Narrative

WE DEPARTED LGA FLYING THE CONEY DEP. ALL PRE-DEP CHKLIST ITEMS HAD BEEN COMPLETELY BRIEFED. BECAUSE OF WT RESTRICTIONS ON RWY 13; THE TKOF REQUIRED A FLAPS 1 SETTING WITH ENG BLEEDS OFF. THIS CONFIGN IS NORMAL; HOWEVER; NOT COMMON. I WAS THE PF. DURING THE INITIAL STAGES OF THE CLBOUT; I CALLED FOR; AND THE CAPT COMPLETED; THE AFTER TKOF CHKLIST; WHICH IS A SILENT CHKLIST. WHILE HE WAS COMPLETING THE CHKLIST; WE RECEIVED SEVERAL ATC INSTRUCTIONS INCLUDING RADIO; ALT; AND HDG CHANGES. WE ALSO ENTERED ICING CONDITIONS REQUIRING THE ANTI-ICING SYS. ALL OF THESE THINGS INTERRUPTED THE FLOW OF THE CHKLIST. AS A RESULT; THE CAPT INADVERTENTLY SHUT DOWN THE APU WITHOUT FIRST TURNING THE ACFT ENG BLEED SWITCHES TO 'ON.' THIS RESULTED IN A GRADUAL RISE IN THE ACFT CABIN ALT. SINCE WE HAD A GRADUAL RISE IN CABIN ALT AND A RAPID CLB RATE; THE 10000 FT WARNING HORN DID NOT SOUND UNTIL WE PASSED 11000 FT. BY THE TIME WE FIGURE OUT WE HAD A PRESSURIZATION PROB; RECEIVED CLRNC AND BEGAN THE PUSHOVER FROM A CLB TO A DSCNT; THE ACFT PASSED THROUGH 14000 FT AND THE PAX OXYGEN MASKS AUTOMATICALLY DEPLOYED. WE REQUESTED AND RECEIVED CLRNC TO RETURN TO LGA WHERE AN UNEVENTFUL LNDG WAS MADE. I BELIEVE THIS SIT OCCURRED AS A RESULT OF SEVERAL CONDITIONS. FIRST; THE NEW EMPHASIS BEING PLACED ON STRICT ADHERENCE TO CHKLIST ITEMS. SECONDLY; THE CHKLIST IS RUN SILENTLY; WHICH LEAVES THE PF OUT OF THE LOOP. FINALLY; THE CONGESTION IN THE NY AREA CAUSES FREQUENT BREAKS IN THE FLOW OF THE CHKLIST DURING CRITICAL PHASES OF FLT.